Coated tablets are well known in the pharmaceutical industry. In addition to pharmaceutical books, manuals and technical literature, patent publications in this field include: U.S. Pat. Nos. 4,543,370 and 4,683,256 on a tablet coating composition of methylcellulose or sodium ethylcellulose sulphate in water or alcohol; Canadian Patent 1,217,140 on a solvent coating composition comprising polyvinyl pyrrolidone, hydroxymethylcellulose, ethylcellulose, sodium lauryl sulphate and propylene glycol; European Patent Application 0 253 541 on a coating with a water permeable but water insoluble ethyl cellulose and dibutyl sebacate.
Aqualon Bulletin VC-556, The Use of Klucel.RTM. Hydroxypropylcellulose, NF, To The Utility of Hydroxpropylmethylcellulose in Aqueous Film Coating, describes how tablets coated from water solutions do not give high gloss such that a polishing coat step is used after the polymer coating in order to improve tablet appearance.
Film coating of pharmaceutical tablets have been carried out for years using organic solvents which provided high gloss finishes. With the move toward the use of water for tablet coatings to avoid the need for solvent recovery systems, it was observed that high gloss tablets could not be produced without the use of organic solvent.